Erenumab is a medicine being tested to help prevent migraines in children and teenagers. A migraine is a type of headache that can be very painful. This study will test if erenumab is safe and works well for kids ages 6 to less than 18 years who have chronic migraines, meaning they have headaches often. The study has four phases, including a screening, a treatment phase where participants get the medicine or a placebo (a "fake" treatment), an optional extension, and a follow-up to check for safety. Kids in the study will get an injection once a month. The study will last for several weeks, and a total of 286 kids will be included.

• Study involves regular visits and injections over several months.
• Participants and parents must agree and understand the process.
• The study may help reduce the number of migraine days.